1. Field of the Invention
The present invention relates to a voltage generating apparatus and to a voltage generating apparatus that supplies a high DC voltage to an electrophotographic image forming apparatus for example.
2. Description of the Related Art
With an electrophotographic image forming apparatus, DC voltage is often used as the charging voltage applied to a charging apparatus, as the developing voltage applied to a developing apparatus, and as the transfer voltage applied to a transfer apparatus. Japanese Patent Laid-Open No. 2000-102249 proposes a high-voltage power source apparatus that supplies stable power to a load whose impedance varies greatly. Depending on the state of the environment, this high-voltage power source apparatus performs at least one of the following: changing the voltage value of the DC voltage that is to be supplied to the primary winding of a step-up transformer (boost transformer), thinning out a pulse width modulation signal generated by a signal generation unit, and changing the frequency of the pulse width modulation signal generated by the signal generation unit.
Japanese Patent Laid-Open No. 9-93920 discloses an invention that achieves a higher charging speed for a capacitive load and stabilizes output such that overshooting and hunting do not occur in a DC high-voltage power source apparatus for a capacitive load. Specifically, the invention disclosed in Japanese Patent Laid-Open No. 9-93920 rapidly charges a load until the output voltage reaches a reference voltage of 90%, and upon reaching 90%, the charging speed is slowed down so as to prevent overshooting and hunting.
The invention disclosed in Japanese Patent Laid-Open No. 2000-102249 might be able to stabilize the output voltage when the load varies greatly due to the environment as well. However, with the method of raising the output by changing the DC voltage on the primary side of the transformer, it is difficult to raise the output voltage at a high speed. This is because a bottleneck occurs in the amount of time needed to charge an electrolytic capacitor arranged on the primary side of the transformer. With the method of thinning out the pulse width modulation signal, ripples tend to increase in size when constant voltage control is performed on the output of a transformer with a high output performance. Furthermore, with the method of changing the frequency of the pulse width modulation signal, there is a risk that the desired output voltage will not be obtained, depending on the characteristics of the transformer. In the invention disclosed in Japanese Patent Laid-Open No. 9-93920, an integrator circuit is arranged on the input side of a circuit that outputs a PWM pulse, and startup time (integration time) is needed when the on-duty width of the PWM pulse is to be raised to the maximum duty width.